Monkeypoxvirus (MPV) is the most virulent human orthopoxvirus infection since the eradication of smallpox, which led to the classification of MPV as a category A virus. The longterm goal of this study is to understand the role of viral immune modulators for MPV virulence and to develop novel treatments and improved vaccine strategies against this emerging pathogen. In 2003, the first MPV outbreak occurred in the US due to MPV imported by pets. Although individuals infected with MPV during this outbreak developed a strong MPV-specific T cell response, we observed that MPV-specific T cells were not stimulated by MPV-infected antigen presenting cells (APC) in vitro. In contrast, infection of APC by the vaccine strain Vaccinia-virus (VV) WR activated cross-reactive CD8+ and CD4+ T cells from MPV-infected individuals. These data strongly suggest that MPV prevents T cell stimulation by expressing immunomodulators that are absent in VV. We recently reported that cowpoxvirus (CPV), which also causes zoonotic infections of humans, similarly prevented the stimulation of CPV-specific CD8+ T cells obtained from experimentally CPV-infected mice. In both CPV and MPV, we observe that CD8+ T cell escape correlates with an inhibition of the maturation of major histocompatibility complex (MHC) class I molecules. Thus, we hypothesize that by inhibiting the presentation of virus-derived peptides to T cells, both MPV and CPV render infected cells invisible to orthopoxvirus-specific CD8+ T cells and potentially CD4+ T cells. In this application we will test this hypothesis by a) determining the molecular mechanism by which MPV downregulates MHC class I molecules, b) examining whether preventing antigen presentation is responsible CD4+ T cell escape, c) identifying the viral inhibitor of antigen presentation (VIPR) of MPV, and d) generating VIPR-deleted MPV and examining whether this recombinant virus activates MPV specific T cells from MPV infected humans and non-human primates (NHP). Upon completion of this exploratory project we plan to address the role of this immune modulatory mechanism for pathogenesis of MPV in NHP and for the ability of MPV to escape vaccine-induced cellular immunity. [unreadable] [unreadable] [unreadable]